Crown boiler's spill switch activates when damper closes. What's up with that?

JohnNY

When the damper closes, heat pours out of the integral draft diverter. Chimney is fine but should be unrelated at that point anyway. No boiler room, but a big wide open drafty basement.
Spill switch is mounted in factory-drilled holes.
I'm thinking this is the second time I've come across this since I had a similar incident a couple years ago with a Crown boiler that used to draft beautifully but shut off in the middle of every night.

What do you think?

hvacfreak2

My first thought is to try a small " delay on break " timer to keep the damper open a little longer.

A.J.

We just had one last month on a Weil McLain boiler . The damper was closing to quickly and not allowing all the combustion byproducts out . We just lock in the damper in the open position . The amount of heat lost to the outside will be minimal .

Mad Dog_2

Seen that before, but not enough to trip the BVS.
Could it be a little over fired and after the gas valve closes, there's just a little too much combustion gas and heat in the chamber? Mad Dog

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Mad Dog_2

For the time being I'd do as AJ said, lock it in manual, but we like to know WHY???? Ahhhh the Quirks of heating....Mad Dog

BPH

what Mad Dog said...to much residual heat build up due to over fire seems possible does it go off on high limit during normal operation running on just the pump? Keeping it simple...weak spill switch?

Gordy

How long is it spilling before it trips the spill switch?

ChrisJ

I'd swap in a new spill switch.

Perhaps it's tripping too low, or the boiler was somehow shipped with the wrong one.

If swapping the spill switch doesn't fix it I'd swap the damper.

I've seen others say the amount of heat lost up a chimney through an atmospheric is minimal, but I don't buy it, I feel a ton of heat gets sucked up the flue during off periods from both through the boiler, as well as through the drafthood and even in a drafty basement that's going to have an effect.

I didn't even want my B-vent open to the basement before the boiler was connected to it due to the amount of draft I felt going up it.

Jason_13

If not a weak spill switch it is a venting issue. The testing on spill switches at the manufacturers is run the boiler, block the vent and it must trip within I believe 10 minutes.
Is there a chimney liner? Is the liner over sized?
Is the chimney taking too long to heat on those colder nights.
Are they turning heat back or using alternate heat source and chimney getting very cold so drafting is an issue when boiler comes back on at night?

Mark Eatherton

Bimetal switches inherently have a high slop factor in their operation. For the time being, take the standing pilot knock out out of the damper blade. I think that might resolve your situation.

ME

captainco

If the boiler was overfired or not venting someone would be deathly sick. On the other hand, without a combustion analyzer those are just guesses and we shouldn't be guessing with someone's safety. With flue gas temperatures well above 400 degrees on a boiler, I am sure it would be easy to trip a spill switch if the flue damper closes too fast. Surprising it happens as little as it does.

Although flue dampers have never been proven to save over 1% or 2% they can prevent severe downdrafts during the off cycle. A delay on the damper would be the first choice and disabling it would be the second choice.

JohnNY

Why would it be a venting issue? The vent is isolated by the closed damper. All but about 2% of it, anyway. This is an 8-10 year old boiler. My guys replaced the spill switch with an OEM 220°F device. I'm going to take the collector off and have a look.

ChrisJ

JohnNY said:

Why would it be a venting issue? The vent is isolated by the closed damper. All but about 2% of it, anyway. This is an 8-10 year old boiler. My guys replaced the spill switch with an OEM 220°F device. I'm going to take the collector off and have a look.

Perhaps if it's running borderline hot while the boiler is in operation due to poor venting, the damper closing is enough to push it over the line?

RayWohlfarth

John we had issues like that It turned out to be downdrafts caused by the wind. The chimney vented just below the top of the roof. We found it as a fluke when I felt the wind blowing in. We changed the chimney outlet and the issues went away. Good luck Nothing like an intermittent problem to test your patience.

Jason_13

Used to work so what changed?
You changed the BVS and still a problem. Still thinking venting or combustion air. This is not a result of the boiler if the switch was changed. If the damper did not open it would not fire. I believe Crowns test is about 3 -4 minutes to trip the BVS. Has there been remodeling done in the home or a new vent fan added in the home? Tightening of the basement?
I had one where there was an elbow on top of the vent damper. When the weather got colder and the boiler fired it would spill where the BVS was. I moved the switch to the opposite side of the vent hood as no spillage there. When the chimney warmed up more the it would no longer spill. The temp of the vent hood on the side that was spilling was over 220f the opposite side was 98f. The chimney spilled about 3-4 minutes. To this day still working good.
Is there a fair amount of vertical pipe above the vent damper or an elbow close to the vent damper?

captainco

If the temperature on one side of the drafthood is that much different than the other side I afraid there is a venting problem with that boiler.

Had a field investigation years ago on a furnace that kept tripping its spill switch but was operating totally safe and just radiating to the spill switch. Looked at another furnace exactly the same in a different room, that was not tripping the spill switch and it was making over 2000ppm of CO and not venting. But it was spilling out the burners not the drafthood because the dilution air wouldn't let the flue gases escape and were keeping the spill switch cool.

347

Is this a steam boiler? If so I had the same problem with a burnham. Found a steam leak under the flue connector and when the damper closed there was too much heat coming out the draft hood which trip the switch, left damper open and problem stopped temporary. Going to replace the boiler soon.